Because GANTTplanner displays the Gantt tasks based on your Google calendar events the active mode depends on what kind of permissions you have for the Google calendar.

If you have read&write permissions then the default mode is active. If you have only read permission (you cannot modify any events in your Google calendar) then the read-only mode is active.

If you would like to change the read-only mode please contact the person which owns the Google calendar. More information on how the owner can change this permission is available in the Google Help Center.